My invention relates to automatic control valves operated either by a modulating electrical or pneumatical signal to control very minute flow rates, as is customary in the operation of pilot plants in the chemical or petroleum industry. Control valves presently in use for this purpose are either of the low-lift variety utilizing a needle-type plug or the longlift cylindrical plug type with precision machined longitudinal scratch, commonly referred to as micro-splined plugs. Both of these types exhibit some problems. The needle-type plug is commonly limited to a tapered angle of 30.degree. to avoid self-locking against the seatring bore. This in turn reduces the stroke whenever small orifice sizes are required. For example, the maximum usable stroke for control with a 1/32 inch orifice is only 0.055 inch, i.e. much too short to achieve any meaningful positioning accuracy with conventional actuating devices. Another draw-back is the very high tendency for fluids to cavitate with needle plugs resulting in early destruction of the valve parts.
The micro-splined plugs require extremely accurate machining of splined grooves having depths of less than 0.001 inch. The minimum controllable area, limiting the lowest flowrate that can be regulated, is given by the radial clearance between the external diameter of the plug and the orifice bore. With a normal radial clearance of 0.00025 inch the minimum controllable area of such a plug with 1/4 inch diameter is 2 .times. 10.sup.-.sup.4 in.sup.2 or 25% of the maximum area of a 1/32 inch dia. orifice, thereby limiting the ratio of max. to min. control area or rangeability to less than 4 : 1.
My invention, on the other hand, greatly reduces the amount of minimum controllable area, since it involves only the product of the radial plug clearance times the circumference of a small bore located perpendicular to the plug axis. For example, with the same radial clearance mentioned in the above example, my minimum controllable area with a 1/32 inch dia. orifice is only 2.4 .times. 10.sup.-.sup.5 in.sup.2, i.e. only 3% of the maximum orifice area.
Additional reductions by one or two order of magnitudes can be made by preloading the plug towards the perpendicular orifice either by utilizing the existing pressure differential or some mechanical means, as explained later on, thereby reducing the radial clearance area i.e. minimum controllable flow area to nearly zero.
A further objection of my invention is the provision of a valve trim with low tendency to cavitate under high pressure liquid flow. My invention accomplishes this goal by having the fluid to undergo a sharp 90.degree. turn from the inlet side (parallel to the cylindrical plug wall) to the perpendicular orifice (outlet port). Such 90.degree. bends cause substantial pressure loss of the fluid and reduce the tendency to cavitate.
One other advantage of my invention is the provision of a valve trim having the advantage to produce high Reynold's numbers, i.e. less effected by viscosity changes in fluids than micro-splined plugs, thereby allowing much more accurate determination of flow capacity.
Finally, my invention provides for a throttling valve trim that can be machined very simply by turning the valve plug tip in shape of a cone, in order to get a very precise and repeatable flow characteristic without resort to unusually precision manufacturing processes.
These and other objections of my invention will become more easily understood from the following detailed description: